Yes, the many US government 3 letter agencies would love to have full read access to every single iPhone in the world. It doesn't mean Apple needs to comply, or that doing so without a search warrant is legal in California
I think you are confusing the rights under US law of US citizens compared to everybody else in the world.
For example, as a New Zealand citizen, I don't expect to have many constitutional rights, nor do I expect I can easily enforce any residual rights I might have using the US justice system (especially against three letter agencies).
Baseless? Police even have access to tools like grayshift/graykey to unlock all but the newest iphones (which inevitably will be supported in time). That is what is known publicly because it is so ubiquitous, plenty of leaks suggest far more sophisticated tools among the FBI let alone agencies with a national security interest.
I would suggest doing some reading on AFU (After First Unlock) and BFU (Before First Unlock) unlocked states. In short, BFU is when you restart your phone without unlocking it: the decryption keys for the user storage are unknown until your credentials are entered. When your device is locked after a restart, you're in AFU mode where the decryption keys are stored in memory.
Devices such as Cellebrite use exploits to extrapolate the decryption key from memory, then use that key on the user storage. This is fundamentally how those tools work. If the device is in BFU, they can't collect nearly as much data.
TL;DR: if you're under threat of having your device taken from you, restart it!
